> I bought Qty 10* of HC520 (12TB) SAS drives for $1000 about 3 weeks ago, avg age is about 2.5 years, still well within its rated lifetime.

That's cheap indeed. Enough headroom for some failing disks too. How is the noise and power usage? I didn't look at SAS drives at all, because my impression was that they're very noisy. I can place my NAS in a closed off room, but it's not too far away and I was afraid SAS drives would be audible through the wall.